| Message ID | 1539965871-22410-3-git-send-email-vincent.guittot@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | sched/fair: update scale invariance of PELT | expand |
Hi Vincent, On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > > /* > + * The clock_pelt scales the time to reflect the effective amount of > + * computation done during the running delta time but then sync back to > + * clock_task when rq is idle. > + * > + * > + * absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 > + * @ max capacity ------******---------------******--------------- > + * @ half capacity ------************---------************--------- > + * clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 > + * > + */ > +void update_rq_clock_pelt(struct rq *rq, s64 delta) > +{ > + > + if (is_idle_task(rq->curr)) { > + u32 divider = (LOAD_AVG_MAX - 1024 + rq->cfs.avg.period_contrib) << SCHED_CAPACITY_SHIFT; > + u32 overload = rq->cfs.avg.util_sum + LOAD_AVG_MAX; > + overload += rq->avg_rt.util_sum; > + overload += rq->avg_dl.util_sum; > + > + /* > + * Reflecting some stolen time makes sense only if the idle > + * phase would be present at max capacity. As soon as the > + * utilization of a rq has reached the maximum value, it is > + * considered as an always runnnig rq without idle time to > + * steal. This potential idle time is considered as lost in > + * this case. We keep track of this lost idle time compare to > + * rq's clock_task. > + */ > + if (overload >= divider) > + rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > + I am trying to understand this better. I believe we run into this scenario, when the frequency is limited due to thermal/userspace constraints. Lets say frequency is limited to Fmax/2. A 50% task at Fmax, becomes 100% running at Fmax/2. The utilization is built up to 100% after several periods. The clock_pelt runs at 1/2 speed of the clock_task. We are loosing the idle time all along. What happens when the CPU enters idle for a short duration and comes back to run this 100% utilization task? If the above block is not present i.e lost_idle_time is not tracked, we stretch the idle time (since clock_pelt is synced to clock_task) and the utilization is dropped. Right? With the above block, we don't stretch the idle time. In fact we don't consider the idle time at all. Because, idle_time = now - last_time; idle_time = (rq->clock_pelt - rq->lost_idle_time) - last_time idle_time = (rq->clock_task - rq_clock_task + rq->clock_pelt_old) - last_time idle_time = rq->clock_pelt_old - last_time The last time is nothing but the last snapshot of the rq->clock_pelt when the task entered sleep due to which CPU entered idle. Can you please explain the significance of the above block with an example? > + > + /* The rq is idle, we can sync to clock_task */ > + rq->clock_pelt = rq_clock_task(rq); > + > + > + } else { > + /* > + * When a rq runs at a lower compute capacity, it will need > + * more time to do the same amount of work than at max > + * capacity: either because it takes more time to compute the > + * same amount of work or because taking more time means > + * sharing more often the CPU between entities. > + * In order to be invariant, we scale the delta to reflect how > + * much work has been really done. > + * Running at lower capacity also means running longer to do > + * the same amount of work and this results in stealing some > + * idle time that will disturb the load signal compared to > + * max capacity; This stolen idle time will be automaticcally > + * reflected when the rq will be idle and the clock will be > + * synced with rq_clock_task. > + */ > + > + /* > + * scale the elapsed time to reflect the real amount of > + * computation > + */ > + delta = cap_scale(delta, arch_scale_freq_capacity(cpu_of(rq))); > + delta = cap_scale(delta, arch_scale_cpu_capacity(NULL, cpu_of(rq))); > + > + rq->clock_pelt += delta; AFAICT, the rq->clock_pelt is used for both utilization and load. So the load also becomes a function of CPU uarch now. Is this intentional? Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > In order to achieve this time scaling, a new clock_pelt is created per rq. > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 3990818..d987f50 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -848,6 +848,8 @@ struct rq { > unsigned int clock_update_flags; > u64 clock; > u64 clock_task; > + u64 clock_pelt; > + unsigned long lost_idle_time; Very clever that. Seems to work out nicely. We should maybe look at ensuring all these clock fields are indeed on the same cacheline.
Hi Pavan, On Tue, 23 Oct 2018 at 07:59, Pavan Kondeti <pkondeti@codeaurora.org> wrote: > > Hi Vincent, > > On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > > > > /* > > + * The clock_pelt scales the time to reflect the effective amount of > > + * computation done during the running delta time but then sync back to > > + * clock_task when rq is idle. > > + * > > + * > > + * absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 > > + * @ max capacity ------******---------------******--------------- > > + * @ half capacity ------************---------************--------- > > + * clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 > > + * > > + */ > > +void update_rq_clock_pelt(struct rq *rq, s64 delta) > > +{ > > + > > + if (is_idle_task(rq->curr)) { > > + u32 divider = (LOAD_AVG_MAX - 1024 + rq->cfs.avg.period_contrib) << SCHED_CAPACITY_SHIFT; > > + u32 overload = rq->cfs.avg.util_sum + LOAD_AVG_MAX; > > + overload += rq->avg_rt.util_sum; > > + overload += rq->avg_dl.util_sum; > > + > > + /* > > + * Reflecting some stolen time makes sense only if the idle > > + * phase would be present at max capacity. As soon as the > > + * utilization of a rq has reached the maximum value, it is > > + * considered as an always runnnig rq without idle time to > > + * steal. This potential idle time is considered as lost in > > + * this case. We keep track of this lost idle time compare to > > + * rq's clock_task. > > + */ > > + if (overload >= divider) > > + rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > > + > > I am trying to understand this better. I believe we run into this scenario, when > the frequency is limited due to thermal/userspace constraints. Lets say Yes these are the most common UCs but this can also happen after tasks migration or with a cpufreq governor that doesn't increase OPP fast enough for current utilization. > frequency is limited to Fmax/2. A 50% task at Fmax, becomes 100% running at > Fmax/2. The utilization is built up to 100% after several periods. > The clock_pelt runs at 1/2 speed of the clock_task. We are loosing the idle time > all along. What happens when the CPU enters idle for a short duration and comes > back to run this 100% utilization task? If you are at 100%, we only apply the short idle duration > > If the above block is not present i.e lost_idle_time is not tracked, we > stretch the idle time (since clock_pelt is synced to clock_task) and the > utilization is dropped. Right? yes that 's what would happen. I gives more details below > > With the above block, we don't stretch the idle time. In fact we don't > consider the idle time at all. Because, > > idle_time = now - last_time; > > idle_time = (rq->clock_pelt - rq->lost_idle_time) - last_time > idle_time = (rq->clock_task - rq_clock_task + rq->clock_pelt_old) - last_time > idle_time = rq->clock_pelt_old - last_time > > The last time is nothing but the last snapshot of the rq->clock_pelt when the > task entered sleep due to which CPU entered idle. The condition for dropping this idle time is quite important. This only happens when the utilization reaches max compute capacity of the CPU. Otherwise, the idle time will be fully applied > > Can you please explain the significance of the above block with an example? The pelt signal reaches its max value after 323ms at full capacity, which means that we can't make any difference between tasks running 323ms, 500ms or more at max capacity. As a result, we consider that the CPU is fully used and there is no idle time when the utilization equals max capacity. If CPU runs at half the capacity, it will run 626ms before reaching max utilization and at that time we will stop to stretch the idle time because we consider that there is no idle time to stretch. By default, we never drop the idle time which is a necessary for being fully invariant and we always apply it. But we have to drop it when we consider that it would not have been present at max capacity too. That's all the purpose of the block that you mention Let take a task that runs 120 ms with a period of 330ms. At max capacity, task utilization will vary in the range [10-949] At half capacity, task will run 240ms and the range will stay the same as the idle time and the running time will be the same once stretched and scaled At one third of the capacity, task should run 360ms in a period of 330 which means that the task will always run and will probably even lost some events as it will have not finished when the new period will start. In this case, the task/CPU utilization will reach the max value just like an always running task. As we can't make any difference anymore, we consider that there is no idle time to recover once the cpu will become idle and the block of code that you mention above will cancel the stretch of idle time. > > > + > > + /* The rq is idle, we can sync to clock_task */ > > + rq->clock_pelt = rq_clock_task(rq); > > + > > + > > + } else { > > + /* > > + * When a rq runs at a lower compute capacity, it will need > > + * more time to do the same amount of work than at max > > + * capacity: either because it takes more time to compute the > > + * same amount of work or because taking more time means > > + * sharing more often the CPU between entities. > > + * In order to be invariant, we scale the delta to reflect how > > + * much work has been really done. > > + * Running at lower capacity also means running longer to do > > + * the same amount of work and this results in stealing some > > + * idle time that will disturb the load signal compared to > > + * max capacity; This stolen idle time will be automaticcally > > + * reflected when the rq will be idle and the clock will be > > + * synced with rq_clock_task. > > + */ > > + > > + /* > > + * scale the elapsed time to reflect the real amount of > > + * computation > > + */ > > + delta = cap_scale(delta, arch_scale_freq_capacity(cpu_of(rq))); > > + delta = cap_scale(delta, arch_scale_cpu_capacity(NULL, cpu_of(rq))); > > + > > + rq->clock_pelt += delta; > > AFAICT, the rq->clock_pelt is used for both utilization and load. So the load > also becomes a function of CPU uarch now. Is this intentional? yes, it is. Load is not scaled with uarch in current implementation because the load would cap by the max capacity of the local CPU and this mess up the load balance. Let take the example of CPU0 with max capacity of 1024 and CPU1 with max capacity of 512. We have 6 always running tasks with same nice priority Then, put 3 tasks on each CPU. If the load is scaled/capped with uarch, LB will consider the system balanced : 3*max_load / 1024 for CPU0 and 3*(max_load / 2) / 512 for CPU1. But tasks on CPU0 have twice more compute capacity than tasks on CPU1. With the new scaling, we don't have this problem anymore so we can take into account uarch and have more accurate load. Regards, Vincent > > Thanks, > Pavan > -- > Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project. >
On Tue, 23 Oct 2018 at 12:01, Peter Zijlstra <peterz@infradead.org> wrote: > > On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > > In order to achieve this time scaling, a new clock_pelt is created per rq. > > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > index 3990818..d987f50 100644 > > --- a/kernel/sched/sched.h > > +++ b/kernel/sched/sched.h > > @@ -848,6 +848,8 @@ struct rq { > > unsigned int clock_update_flags; > > u64 clock; > > u64 clock_task; > > + u64 clock_pelt; > > + unsigned long lost_idle_time; > > Very clever that. Seems to work out nicely. We should maybe look at Thanks > ensuring all these clock fields are indeed on the same cacheline. yes good point
Hi Vincent, Thanks for the detailed explanation. On Tue, Oct 23, 2018 at 02:15:08PM +0200, Vincent Guittot wrote: > Hi Pavan, > > On Tue, 23 Oct 2018 at 07:59, Pavan Kondeti <pkondeti@codeaurora.org> wrote: > > > > Hi Vincent, > > > > On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > > > > > > /* > > > + * The clock_pelt scales the time to reflect the effective amount of > > > + * computation done during the running delta time but then sync back to > > > + * clock_task when rq is idle. > > > + * > > > + * > > > + * absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 > > > + * @ max capacity ------******---------------******--------------- > > > + * @ half capacity ------************---------************--------- > > > + * clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 > > > + * > > > + */ > > > +void update_rq_clock_pelt(struct rq *rq, s64 delta) > > > +{ > > > + > > > + if (is_idle_task(rq->curr)) { > > > + u32 divider = (LOAD_AVG_MAX - 1024 + rq->cfs.avg.period_contrib) << SCHED_CAPACITY_SHIFT; > > > + u32 overload = rq->cfs.avg.util_sum + LOAD_AVG_MAX; > > > + overload += rq->avg_rt.util_sum; > > > + overload += rq->avg_dl.util_sum; > > > + > > > + /* > > > + * Reflecting some stolen time makes sense only if the idle > > > + * phase would be present at max capacity. As soon as the > > > + * utilization of a rq has reached the maximum value, it is > > > + * considered as an always runnnig rq without idle time to > > > + * steal. This potential idle time is considered as lost in > > > + * this case. We keep track of this lost idle time compare to > > > + * rq's clock_task. > > > + */ > > > + if (overload >= divider) > > > + rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > > > + > > > > I am trying to understand this better. I believe we run into this scenario, when > > the frequency is limited due to thermal/userspace constraints. Lets say > > Yes these are the most common UCs but this can also happen after tasks > migration or with a cpufreq governor that doesn't increase OPP fast > enough for current utilization. > > > frequency is limited to Fmax/2. A 50% task at Fmax, becomes 100% running at > > Fmax/2. The utilization is built up to 100% after several periods. > > The clock_pelt runs at 1/2 speed of the clock_task. We are loosing the idle time > > all along. What happens when the CPU enters idle for a short duration and comes > > back to run this 100% utilization task? > > If you are at 100%, we only apply the short idle duration > > > > > If the above block is not present i.e lost_idle_time is not tracked, we > > stretch the idle time (since clock_pelt is synced to clock_task) and the > > utilization is dropped. Right? > > yes that 's what would happen. I gives more details below > > > > > With the above block, we don't stretch the idle time. In fact we don't > > consider the idle time at all. Because, > > > > idle_time = now - last_time; > > > > idle_time = (rq->clock_pelt - rq->lost_idle_time) - last_time > > idle_time = (rq->clock_task - rq_clock_task + rq->clock_pelt_old) - last_time > > idle_time = rq->clock_pelt_old - last_time > > > > The last time is nothing but the last snapshot of the rq->clock_pelt when the > > task entered sleep due to which CPU entered idle. > > The condition for dropping this idle time is quite important. This > only happens when the utilization reaches max compute capacity of the > CPU. Otherwise, the idle time will be fully applied Right. rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt This not only tracks the lost idle time due to running slow but also the absolute/real sleep time. For example, when the slow running 100% task sleeps for 100 msec, are not we ignoring the 100 msec sleep there? For example a task ran 323 msec at full capacity and sleeps for (1000-323) msec. when it wakes up the utilization is dropped. If the same task runs for 626 msec at the half capacity and sleeps for (1000-626), should not drop the utilization by taking (1000-626) sleep time into account. I understand that why we don't strech idle time to (1000-323) but it is not clear to me why we completely drop the idle time. > > > > > Can you please explain the significance of the above block with an example? > > The pelt signal reaches its max value after 323ms at full capacity, > which means that we can't make any difference between tasks running > 323ms, 500ms or more at max capacity. As a result, we consider that > the CPU is fully used and there is no idle time when the utilization > equals max capacity. If CPU runs at half the capacity, it will run > 626ms before reaching max utilization and at that time we will stop to > stretch the idle time because we consider that there is no idle time > to stretch. By default, we never drop the idle time which is a > necessary for being fully invariant and we always apply it. But we > have to drop it when we consider that it would not have been present > at max capacity too. That's all the purpose of the block that you > mention This is very much clear. > > Let take a task that runs 120 ms with a period of 330ms. > At max capacity, task utilization will vary in the range [10-949] > At half capacity, task will run 240ms and the range will stay the same > as the idle time and the running time will be the same once stretched > and scaled > At one third of the capacity, task should run 360ms in a period of 330 > which means that the task will always run and will probably even lost > some events as it will have not finished when the new period will > start. In this case, the task/CPU utilization will reach the max value > just like an always running task. As we can't make any difference > anymore, we consider that there is no idle time to recover once the > cpu will become idle and the block of code that you mention above will > cancel the stretch of idle time. > Got it. > > > > > + > > > + /* The rq is idle, we can sync to clock_task */ > > > + rq->clock_pelt = rq_clock_task(rq); > > > + > > > + > > > + } else { > > > + /* > > > + * When a rq runs at a lower compute capacity, it will need > > > + * more time to do the same amount of work than at max > > > + * capacity: either because it takes more time to compute the > > > + * same amount of work or because taking more time means > > > + * sharing more often the CPU between entities. > > > + * In order to be invariant, we scale the delta to reflect how > > > + * much work has been really done. > > > + * Running at lower capacity also means running longer to do > > > + * the same amount of work and this results in stealing some > > > + * idle time that will disturb the load signal compared to > > > + * max capacity; This stolen idle time will be automaticcally > > > + * reflected when the rq will be idle and the clock will be > > > + * synced with rq_clock_task. > > > + */ > > > + > > > + /* > > > + * scale the elapsed time to reflect the real amount of > > > + * computation > > > + */ > > > + delta = cap_scale(delta, arch_scale_freq_capacity(cpu_of(rq))); > > > + delta = cap_scale(delta, arch_scale_cpu_capacity(NULL, cpu_of(rq))); > > > + > > > + rq->clock_pelt += delta; > > > > AFAICT, the rq->clock_pelt is used for both utilization and load. So the load > > also becomes a function of CPU uarch now. Is this intentional? > > yes, it is. Load is not scaled with uarch in current implementation > because the load would cap by the max capacity of the local CPU and > this mess up the load balance. > > Let take the example of CPU0 with max capacity of 1024 and CPU1 with > max capacity of 512. > We have 6 always running tasks with same nice priority > Then, put 3 tasks on each CPU. > If the load is scaled/capped with uarch, LB will consider the system > balanced : 3*max_load / 1024 for CPU0 and 3*(max_load / 2) / 512 for > CPU1. But tasks on CPU0 have twice more compute capacity than tasks on > CPU1. > > With the new scaling, we don't have this problem anymore so we can > take into account uarch and have more accurate load. > Got it. Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
Hi Pavan, On Wed, 24 Oct 2018 at 06:53, Pavan Kondeti <pkondeti@codeaurora.org> wrote: > > Hi Vincent, > > Thanks for the detailed explanation. > > On Tue, Oct 23, 2018 at 02:15:08PM +0200, Vincent Guittot wrote: > > Hi Pavan, > > > > On Tue, 23 Oct 2018 at 07:59, Pavan Kondeti <pkondeti@codeaurora.org> wrote: > > > > > > Hi Vincent, > > > > > > On Fri, Oct 19, 2018 at 06:17:51PM +0200, Vincent Guittot wrote: > > > > > > > > /* > > > > + * The clock_pelt scales the time to reflect the effective amount of > > > > + * computation done during the running delta time but then sync back to > > > > + * clock_task when rq is idle. > > > > + * > > > > + * > > > > + * absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 > > > > + * @ max capacity ------******---------------******--------------- > > > > + * @ half capacity ------************---------************--------- > > > > + * clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 > > > > + * > > > > + */ > > > > +void update_rq_clock_pelt(struct rq *rq, s64 delta) > > > > +{ > > > > + > > > > + if (is_idle_task(rq->curr)) { > > > > + u32 divider = (LOAD_AVG_MAX - 1024 + rq->cfs.avg.period_contrib) << SCHED_CAPACITY_SHIFT; > > > > + u32 overload = rq->cfs.avg.util_sum + LOAD_AVG_MAX; > > > > + overload += rq->avg_rt.util_sum; > > > > + overload += rq->avg_dl.util_sum; > > > > + > > > > + /* > > > > + * Reflecting some stolen time makes sense only if the idle > > > > + * phase would be present at max capacity. As soon as the > > > > + * utilization of a rq has reached the maximum value, it is > > > > + * considered as an always runnnig rq without idle time to > > > > + * steal. This potential idle time is considered as lost in > > > > + * this case. We keep track of this lost idle time compare to > > > > + * rq's clock_task. > > > > + */ > > > > + if (overload >= divider) > > > > + rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > > > > + > > > > > > I am trying to understand this better. I believe we run into this scenario, when > > > the frequency is limited due to thermal/userspace constraints. Lets say > > > > Yes these are the most common UCs but this can also happen after tasks > > migration or with a cpufreq governor that doesn't increase OPP fast > > enough for current utilization. > > > > > frequency is limited to Fmax/2. A 50% task at Fmax, becomes 100% running at > > > Fmax/2. The utilization is built up to 100% after several periods. > > > The clock_pelt runs at 1/2 speed of the clock_task. We are loosing the idle time > > > all along. What happens when the CPU enters idle for a short duration and comes > > > back to run this 100% utilization task? > > > > If you are at 100%, we only apply the short idle duration > > > > > > > > If the above block is not present i.e lost_idle_time is not tracked, we > > > stretch the idle time (since clock_pelt is synced to clock_task) and the > > > utilization is dropped. Right? > > > > yes that 's what would happen. I gives more details below > > > > > > > > With the above block, we don't stretch the idle time. In fact we don't > > > consider the idle time at all. Because, > > > > > > idle_time = now - last_time; > > > > > > idle_time = (rq->clock_pelt - rq->lost_idle_time) - last_time > > > idle_time = (rq->clock_task - rq_clock_task + rq->clock_pelt_old) - last_time > > > idle_time = rq->clock_pelt_old - last_time > > > > > > The last time is nothing but the last snapshot of the rq->clock_pelt when the > > > task entered sleep due to which CPU entered idle. > > > > The condition for dropping this idle time is quite important. This > > only happens when the utilization reaches max compute capacity of the > > CPU. Otherwise, the idle time will be fully applied > > Right. > > rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt > > This not only tracks the lost idle time due to running slow but also the > absolute/real sleep time. For example, when the slow running 100% task > sleeps for 100 msec, are not we ignoring the 100 msec sleep there? > > For example a task ran 323 msec at full capacity and sleeps for (1000-323) > msec. when it wakes up the utilization is dropped. If the same task runs > for 626 msec at the half capacity and sleeps for (1000-626), should not > drop the utilization by taking (1000-626) sleep time into account. I > understand that why we don't strech idle time to (1000-323) but it is not > clear to me why we completely drop the idle time. So this should not happen. I' m going to update the way I track lost idle time and move it out of update_rq_clock_pelt() and only do the test when entering idle This is even better as it simplifies update_rq_clock_pelt() and reduces the number of tests for lost idle time Thanks for spotting this I'm preparing a new version with this, some build fix for !SMP and the alignement with cache line suggested by Peter Vincent > > > > > >
Hi Vincent, On 10/19/18 6:17 PM, Vincent Guittot wrote: > The current implementation of load tracking invariance scales the > contribution with current frequency and uarch performance (only for > utilization) of the CPU. One main result of this formula is that the > figures are capped by current capacity of CPU. Another one is that the > load_avg is not invariant because not scaled with uarch. > > The util_avg of a periodic task that runs r time slots every p time slots > varies in the range : > > U * (1-y^r)/(1-y^p) * y^i < Utilization < U * (1-y^r)/(1-y^p) > > with U is the max util_avg value = SCHED_CAPACITY_SCALE > > At a lower capacity, the range becomes: > > U * C * (1-y^r')/(1-y^p) * y^i' < Utilization < U * C * (1-y^r')/(1-y^p) > > with C reflecting the compute capacity ratio between current capacity and > max capacity. > > so C tries to compensate changes in (1-y^r') but it can't be accurate. > > Instead of scaling the contribution value of PELT algo, we should scale the > running time. The PELT signal aims to track the amount of computation of > tasks and/or rq so it seems more correct to scale the running time to > reflect the effective amount of computation done since the last update. > > In order to be fully invariant, we need to apply the same amount of > running time and idle time whatever the current capacity. Because running > at lower capacity implies that the task will run longer, we have to ensure > that the same amount of idle time will be apply when system becomes idle > and no idle time has been "stolen". But reaching the maximum utilization > value (SCHED_CAPACITY_SCALE) means that the task is seen as an > always-running task whatever the capacity of the CPU (even at max compute > capacity). In this case, we can discard this "stolen" idle times which > becomes meaningless. > > In order to achieve this time scaling, a new clock_pelt is created per rq. > The increase of this clock scales with current capacity when something > is running on rq and synchronizes with clock_task when rq is idle. With > this mecanism, we ensure the same running and idle time whatever the > current capacity. This also enables to simplify the pelt algorithm by > removing all references of uarch and frequency and applying the same > contribution to utilization and loads. Furthermore, the scaling is done > only once per update of clock (update_rq_clock_task()) instead of during > each update of sched_entities and cfs/rt/dl_rq of the rq like the current > implementation. This is interesting when cgroup are involved as shown in > the results below: I have a couple of questions related to the tests you ran. > On a hikey (octo ARM platform). > Performance cpufreq governor and only shallowest c-state to remove variance > generated by those power features so we only track the impact of pelt algo. So you disabled c-state 'cpu-sleep' and 'cluster-sleep'? I get 'hisi_thermal f7030700.tsensor: THERMAL ALARM: 66385 > 65000' on my hikey620. Did you change the thermal configuration? Not sure if there are any actions attached to this warning though. > each test runs 16 times > > ./perf bench sched pipe > (higher is better) > kernel tip/sched/core + patch > ops/seconds ops/seconds diff > cgroup > root 59648(+/- 0.13%) 59785(+/- 0.24%) +0.23% > level1 55570(+/- 0.21%) 56003(+/- 0.24%) +0.78% > level2 52100(+/- 0.20%) 52788(+/- 0.22%) +1.32% > > hackbench -l 1000 Shouldn't this be '-l 100'? > (lower is better) > kernel tip/sched/core + patch > duration(sec) duration(sec) diff > cgroup > root 4.472(+/- 1.86%) 4.346(+/- 2.74%) -2.80% > level1 5.039(+/- 11.05%) 4.662(+/- 7.57%) -7.47% > level2 5.195(+/- 10.66%) 4.877(+/- 8.90%) -6.12% > > The responsivness of PELT is improved when CPU is not running at max > capacity with this new algorithm. I have put below some examples of > duration to reach some typical load values according to the capacity of the > CPU with current implementation and with this patch. > > Util (%) max capacity half capacity(mainline) half capacity(w/ patch) > 972 (95%) 138ms not reachable 276ms > 486 (47.5%) 30ms 138ms 60ms > 256 (25%) 13ms 32ms 26ms Could you describe these testcases in more detail? So I assume you run one 100% task (possibly pinned to one CPU) on your hikey620 with userspace governor and for: (1) max capacity: echo 1200000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed (2) half capacity: echo 729000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed and then you measure the time till t1 reaches 25%, 47.5% and 95% utilization? What's the initial utilization value of t1? I assume t1 starts with utilization=512 (post_init_entity_util_avg()). > On my hikey (octo ARM platform) with schedutil governor, the time to reach > max OPP when starting from a null utilization, decreases from 223ms with > current scale invariance down to 121ms with the new algorithm. For this > test, I have enable arch_scale_freq for arm64. Isn't the arch-specific arch_scale_freq_capacity() enabled by default on arm64 with cpufreq support? I would like to run the same tests so we can discuss results more easily.
On Thu, 25 Oct 2018 at 12:36, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > Hi Vincent, > > On 10/19/18 6:17 PM, Vincent Guittot wrote: > > The current implementation of load tracking invariance scales the > > contribution with current frequency and uarch performance (only for > > utilization) of the CPU. One main result of this formula is that the > > figures are capped by current capacity of CPU. Another one is that the > > load_avg is not invariant because not scaled with uarch. > > > > The util_avg of a periodic task that runs r time slots every p time slots > > varies in the range : > > > > U * (1-y^r)/(1-y^p) * y^i < Utilization < U * (1-y^r)/(1-y^p) > > > > with U is the max util_avg value = SCHED_CAPACITY_SCALE > > > > At a lower capacity, the range becomes: > > > > U * C * (1-y^r')/(1-y^p) * y^i' < Utilization < U * C * (1-y^r')/(1-y^p) > > > > with C reflecting the compute capacity ratio between current capacity and > > max capacity. > > > > so C tries to compensate changes in (1-y^r') but it can't be accurate. > > > > Instead of scaling the contribution value of PELT algo, we should scale the > > running time. The PELT signal aims to track the amount of computation of > > tasks and/or rq so it seems more correct to scale the running time to > > reflect the effective amount of computation done since the last update. > > > > In order to be fully invariant, we need to apply the same amount of > > running time and idle time whatever the current capacity. Because running > > at lower capacity implies that the task will run longer, we have to ensure > > that the same amount of idle time will be apply when system becomes idle > > and no idle time has been "stolen". But reaching the maximum utilization > > value (SCHED_CAPACITY_SCALE) means that the task is seen as an > > always-running task whatever the capacity of the CPU (even at max compute > > capacity). In this case, we can discard this "stolen" idle times which > > becomes meaningless. > > > > In order to achieve this time scaling, a new clock_pelt is created per rq. > > The increase of this clock scales with current capacity when something > > is running on rq and synchronizes with clock_task when rq is idle. With > > this mecanism, we ensure the same running and idle time whatever the > > current capacity. This also enables to simplify the pelt algorithm by > > removing all references of uarch and frequency and applying the same > > contribution to utilization and loads. Furthermore, the scaling is done > > only once per update of clock (update_rq_clock_task()) instead of during > > each update of sched_entities and cfs/rt/dl_rq of the rq like the current > > implementation. This is interesting when cgroup are involved as shown in > > the results below: > > I have a couple of questions related to the tests you ran. > > > On a hikey (octo ARM platform). > > Performance cpufreq governor and only shallowest c-state to remove variance > > generated by those power features so we only track the impact of pelt algo. > > So you disabled c-state 'cpu-sleep' and 'cluster-sleep'? yes > > I get 'hisi_thermal f7030700.tsensor: THERMAL ALARM: 66385 > 65000' on > my hikey620. Did you change the thermal configuration? Not sure if there > are any actions attached to this warning though. I have a fan to ensure that no thermal mitigation will bias the measurement. > > > each test runs 16 times > > > > ./perf bench sched pipe > > (higher is better) > > kernel tip/sched/core + patch > > ops/seconds ops/seconds diff > > cgroup > > root 59648(+/- 0.13%) 59785(+/- 0.24%) +0.23% > > level1 55570(+/- 0.21%) 56003(+/- 0.24%) +0.78% > > level2 52100(+/- 0.20%) 52788(+/- 0.22%) +1.32% > > > > hackbench -l 1000 > > Shouldn't this be '-l 100'? I have re checked and it's -l 1000 > > > (lower is better) > > kernel tip/sched/core + patch > > duration(sec) duration(sec) diff > > cgroup > > root 4.472(+/- 1.86%) 4.346(+/- 2.74%) -2.80% > > level1 5.039(+/- 11.05%) 4.662(+/- 7.57%) -7.47% > > level2 5.195(+/- 10.66%) 4.877(+/- 8.90%) -6.12% > > > > The responsivness of PELT is improved when CPU is not running at max > > capacity with this new algorithm. I have put below some examples of > > duration to reach some typical load values according to the capacity of the > > CPU with current implementation and with this patch. > > > > Util (%) max capacity half capacity(mainline) half capacity(w/ patch) > > 972 (95%) 138ms not reachable 276ms > > 486 (47.5%) 30ms 138ms 60ms > > 256 (25%) 13ms 32ms 26ms > > Could you describe these testcases in more detail? You don't need to run test case. These numbers are computed based on geometric series and half period value > > So I assume you run one 100% task (possibly pinned to one CPU) on your > hikey620 with userspace governor and for: > > (1) max capacity: > > echo 1200000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed > > (2) half capacity: > > echo 729000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed > > and then you measure the time till t1 reaches 25%, 47.5% and 95% > utilization? > What's the initial utilization value of t1? I assume t1 starts with > utilization=512 (post_init_entity_util_avg()). > > > On my hikey (octo ARM platform) with schedutil governor, the time to reach > > max OPP when starting from a null utilization, decreases from 223ms with > > current scale invariance down to 121ms with the new algorithm. For this > > test, I have enable arch_scale_freq for arm64. > > Isn't the arch-specific arch_scale_freq_capacity() enabled by default on > arm64 with cpufreq support? Yes. that's a remain of previous version when arch_scale_freq was not yet merged > > I would like to run the same tests so we can discuss results more easily. Let me know if you need more details
On 10/25/18 12:43 PM, Vincent Guittot wrote: > On Thu, 25 Oct 2018 at 12:36, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: [...] >> I have a couple of questions related to the tests you ran. >> >>> On a hikey (octo ARM platform). >>> Performance cpufreq governor and only shallowest c-state to remove variance >>> generated by those power features so we only track the impact of pelt algo. >> >> So you disabled c-state 'cpu-sleep' and 'cluster-sleep'? > > yes > >> >> I get 'hisi_thermal f7030700.tsensor: THERMAL ALARM: 66385 > 65000' on >> my hikey620. Did you change the thermal configuration? Not sure if there >> are any actions attached to this warning though. > > I have a fan to ensure that no thermal mitigation will bias the measurement. Great, with a fan they disappear here as well. >>> each test runs 16 times >>> >>> ./perf bench sched pipe >>> (higher is better) >>> kernel tip/sched/core + patch >>> ops/seconds ops/seconds diff >>> cgroup >>> root 59648(+/- 0.13%) 59785(+/- 0.24%) +0.23% >>> level1 55570(+/- 0.21%) 56003(+/- 0.24%) +0.78% >>> level2 52100(+/- 0.20%) 52788(+/- 0.22%) +1.32% >>> >>> hackbench -l 1000 >> >> Shouldn't this be '-l 100'? > > I have re checked and it's -l 1000 Strange, when I run hackbench on this board (performance governor) I get values like: root@h620:/# hackbench -l 100 Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks) Each sender will pass 100 messages of 100 bytes Time: 4.023 root@h620:/# hackbench -l 1000 Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks) Each sender will pass 1000 messages of 100 bytes Time: 37.883 Since you have values in the range of 4-6 secs in your hackbench table? Maybe different hackbench versions? >>> (lower is better) >>> kernel tip/sched/core + patch >>> duration(sec) duration(sec) diff >>> cgroup >>> root 4.472(+/- 1.86%) 4.346(+/- 2.74%) -2.80% >>> level1 5.039(+/- 11.05%) 4.662(+/- 7.57%) -7.47% >>> level2 5.195(+/- 10.66%) 4.877(+/- 8.90%) -6.12% >>> >>> The responsivness of PELT is improved when CPU is not running at max >>> capacity with this new algorithm. I have put below some examples of >>> duration to reach some typical load values according to the capacity of the >>> CPU with current implementation and with this patch. >>> >>> Util (%) max capacity half capacity(mainline) half capacity(w/ patch) >>> 972 (95%) 138ms not reachable 276ms >>> 486 (47.5%) 30ms 138ms 60ms >>> 256 (25%) 13ms 32ms 26ms >> >> Could you describe these testcases in more detail? > > You don't need to run test case. These numbers are computed based on > geometric series and half period value Ah, ok, maybe you can mention this explicitly. [...] >> What's the initial utilization value of t1? I assume t1 starts with >> utilization=512 (post_init_entity_util_avg()). OK, then it's starts at 0. >>> On my hikey (octo ARM platform) with schedutil governor, the time to reach >>> max OPP when starting from a null utilization, decreases from 223ms with >>> current scale invariance down to 121ms with the new algorithm. For this >>> test, I have enable arch_scale_freq for arm64. >> >> Isn't the arch-specific arch_scale_freq_capacity() enabled by default on >> arm64 with cpufreq support? > > Yes. that's a remain of previous version when arch_scale_freq was not yet merged OK. [...]
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 625bc98..84e5c48 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -181,6 +181,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta) if ((irq_delta + steal) && sched_feat(NONTASK_CAPACITY)) update_irq_load_avg(rq, irq_delta + steal); #endif + update_rq_clock_pelt(rq, delta); } void update_rq_clock(struct rq *rq) @@ -205,7 +206,6 @@ void update_rq_clock(struct rq *rq) update_rq_clock_task(rq, delta); } - #ifdef CONFIG_SCHED_HRTICK /* * Use HR-timers to deliver accurate preemption points. diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 997ea7b..68cb4dc 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1761,7 +1761,7 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) deadline_queue_push_tasks(rq); if (rq->curr->sched_class != &dl_sched_class) - update_dl_rq_load_avg(rq_clock_task(rq), rq, 0); + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 0); return p; } @@ -1770,7 +1770,7 @@ static void put_prev_task_dl(struct rq *rq, struct task_struct *p) { update_curr_dl(rq); - update_dl_rq_load_avg(rq_clock_task(rq), rq, 1); + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1); if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1) enqueue_pushable_dl_task(rq, p); } @@ -1787,7 +1787,7 @@ static void task_tick_dl(struct rq *rq, struct task_struct *p, int queued) { update_curr_dl(rq); - update_dl_rq_load_avg(rq_clock_task(rq), rq, 1); + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1); /* * Even when we have runtime, update_curr_dl() might have resulted in us * not being the leftmost task anymore. In that case NEED_RESCHED will diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0969ce3..5677254 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -764,7 +764,7 @@ void post_init_entity_util_avg(struct sched_entity *se) * such that the next switched_to_fair() has the * expected state. */ - se->avg.last_update_time = cfs_rq_clock_task(cfs_rq); + se->avg.last_update_time = cfs_rq_clock_pelt(cfs_rq); return; } } @@ -3400,7 +3400,7 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s /* Update task and its cfs_rq load average */ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) { - u64 now = cfs_rq_clock_task(cfs_rq); + u64 now = cfs_rq_clock_pelt(cfs_rq); struct rq *rq = rq_of(cfs_rq); int cpu = cpu_of(rq); int decayed; @@ -7285,7 +7285,7 @@ static void update_blocked_averages(int cpu) if (throttled_hierarchy(cfs_rq)) continue; - if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq)) + if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) update_tg_load_avg(cfs_rq, 0); /* Propagate pending load changes to the parent, if any: */ @@ -7306,8 +7306,8 @@ static void update_blocked_averages(int cpu) } curr_class = rq->curr->sched_class; - update_rt_rq_load_avg(rq_clock_task(rq), rq, curr_class == &rt_sched_class); - update_dl_rq_load_avg(rq_clock_task(rq), rq, curr_class == &dl_sched_class); + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class); + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class); update_irq_load_avg(rq, 0); /* Don't need periodic decay once load/util_avg are null */ if (others_have_blocked(rq)) @@ -7377,11 +7377,11 @@ static inline void update_blocked_averages(int cpu) rq_lock_irqsave(rq, &rf); update_rq_clock(rq); - update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq); + update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq); curr_class = rq->curr->sched_class; - update_rt_rq_load_avg(rq_clock_task(rq), rq, curr_class == &rt_sched_class); - update_dl_rq_load_avg(rq_clock_task(rq), rq, curr_class == &dl_sched_class); + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class); + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class); update_irq_load_avg(rq, 0); #ifdef CONFIG_NO_HZ_COMMON rq->last_blocked_load_update_tick = jiffies; diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c index 35475c0..48f4f07 100644 --- a/kernel/sched/pelt.c +++ b/kernel/sched/pelt.c @@ -30,6 +30,72 @@ #include "pelt.h" /* + * The clock_pelt scales the time to reflect the effective amount of + * computation done during the running delta time but then sync back to + * clock_task when rq is idle. + * + * + * absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 + * @ max capacity ------******---------------******--------------- + * @ half capacity ------************---------************--------- + * clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 + * + */ +void update_rq_clock_pelt(struct rq *rq, s64 delta) +{ + + if (is_idle_task(rq->curr)) { + u32 divider = (LOAD_AVG_MAX - 1024 + rq->cfs.avg.period_contrib) << SCHED_CAPACITY_SHIFT; + u32 overload = rq->cfs.avg.util_sum + LOAD_AVG_MAX; + overload += rq->avg_rt.util_sum; + overload += rq->avg_dl.util_sum; + + /* + * Reflecting some stolen time makes sense only if the idle + * phase would be present at max capacity. As soon as the + * utilization of a rq has reached the maximum value, it is + * considered as an always runnnig rq without idle time to + * steal. This potential idle time is considered as lost in + * this case. We keep track of this lost idle time compare to + * rq's clock_task. + */ + if (overload >= divider) + rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; + + + /* The rq is idle, we can sync to clock_task */ + rq->clock_pelt = rq_clock_task(rq); + + + } else { + /* + * When a rq runs at a lower compute capacity, it will need + * more time to do the same amount of work than at max + * capacity: either because it takes more time to compute the + * same amount of work or because taking more time means + * sharing more often the CPU between entities. + * In order to be invariant, we scale the delta to reflect how + * much work has been really done. + * Running at lower capacity also means running longer to do + * the same amount of work and this results in stealing some + * idle time that will disturb the load signal compared to + * max capacity; This stolen idle time will be automaticcally + * reflected when the rq will be idle and the clock will be + * synced with rq_clock_task. + */ + + /* + * scale the elapsed time to reflect the real amount of + * computation + */ + delta = cap_scale(delta, arch_scale_freq_capacity(cpu_of(rq))); + delta = cap_scale(delta, arch_scale_cpu_capacity(NULL, cpu_of(rq))); + + rq->clock_pelt += delta; + } +} + +/* * Approximate: * val * y^n, where y^32 ~= 0.5 (~1 scheduling period) */ @@ -106,16 +172,12 @@ static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3) * n=1 */ static __always_inline u32 -accumulate_sum(u64 delta, int cpu, struct sched_avg *sa, +accumulate_sum(u64 delta, struct sched_avg *sa, unsigned long load, unsigned long runnable, int running) { - unsigned long scale_freq, scale_cpu; u32 contrib = (u32)delta; /* p == 0 -> delta < 1024 */ u64 periods; - scale_freq = arch_scale_freq_capacity(cpu); - scale_cpu = arch_scale_cpu_capacity(NULL, cpu); - delta += sa->period_contrib; periods = delta / 1024; /* A period is 1024us (~1ms) */ @@ -137,13 +199,12 @@ accumulate_sum(u64 delta, int cpu, struct sched_avg *sa, } sa->period_contrib = delta; - contrib = cap_scale(contrib, scale_freq); if (load) sa->load_sum += load * contrib; if (runnable) sa->runnable_load_sum += runnable * contrib; if (running) - sa->util_sum += contrib * scale_cpu; + sa->util_sum += contrib << SCHED_CAPACITY_SHIFT; return periods; } @@ -221,7 +282,7 @@ ___update_load_sum(u64 now, int cpu, struct sched_avg *sa, * Step 1: accumulate *_sum since last_update_time. If we haven't * crossed period boundaries, finish. */ - if (!accumulate_sum(delta, cpu, sa, load, runnable, running)) + if (!accumulate_sum(delta, sa, load, runnable, running)) return 0; return 1; @@ -371,12 +432,21 @@ int update_dl_rq_load_avg(u64 now, struct rq *rq, int running) int update_irq_load_avg(struct rq *rq, u64 running) { int ret = 0; + + /* + * We can't use clock_pelt because irq time is not accounted in + * clock_task. Instead we directly scale the running time to + * reflect the real amount of computation + */ + running = cap_scale(running, arch_scale_freq_capacity(cpu_of(rq))); + running = cap_scale(running, arch_scale_cpu_capacity(NULL, cpu_of(rq))); + /* * We know the time that has been used by interrupt since last update * but we don't when. Let be pessimistic and assume that interrupt has * happened just before the update. This is not so far from reality * because interrupt will most probably wake up task and trig an update - * of rq clock during which the metric si updated. + * of rq clock during which the metric is updated. * We start to decay with normal context time and then we add the * interrupt context time. * We can safely remove running from rq->clock because diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h index d2894db..b4ce173 100644 --- a/kernel/sched/pelt.h +++ b/kernel/sched/pelt.h @@ -42,6 +42,29 @@ static inline void cfs_se_util_change(struct sched_avg *avg) WRITE_ONCE(avg->util_est.enqueued, enqueued); } +void update_rq_clock_pelt(struct rq *rq, s64 delta); + +static inline u64 rq_clock_pelt(struct rq *rq) +{ + return rq->clock_pelt - rq->lost_idle_time; +} + +#ifdef CONFIG_CFS_BANDWIDTH +/* rq->task_clock normalized against any time this cfs_rq has spent throttled */ +static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) +{ + if (unlikely(cfs_rq->throttle_count)) + return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time; + + return rq_clock_pelt(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time; +} +#else +static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) +{ + return rq_clock_pelt(rq_of(cfs_rq)); +} +#endif + #else static inline int @@ -67,6 +90,10 @@ update_irq_load_avg(struct rq *rq, u64 running) { return 0; } + +static inline void +update_rq_clock_pelt(struct rq *rq, s64 delta) {} + #endif diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 2e2955a..f62f2d5 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1584,7 +1584,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) * rt task */ if (rq->curr->sched_class != &rt_sched_class) - update_rt_rq_load_avg(rq_clock_task(rq), rq, 0); + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0); return p; } @@ -1593,7 +1593,7 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p) { update_curr_rt(rq); - update_rt_rq_load_avg(rq_clock_task(rq), rq, 1); + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1); /* * The previous task needs to be made eligible for pushing @@ -2324,7 +2324,7 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued) struct sched_rt_entity *rt_se = &p->rt; update_curr_rt(rq); - update_rt_rq_load_avg(rq_clock_task(rq), rq, 1); + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1); watchdog(rq, p); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 3990818..d987f50 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -848,6 +848,8 @@ struct rq { unsigned int clock_update_flags; u64 clock; u64 clock_task; + u64 clock_pelt; + unsigned long lost_idle_time; atomic_t nr_iowait;
The current implementation of load tracking invariance scales the contribution with current frequency and uarch performance (only for utilization) of the CPU. One main result of this formula is that the figures are capped by current capacity of CPU. Another one is that the load_avg is not invariant because not scaled with uarch. The util_avg of a periodic task that runs r time slots every p time slots varies in the range : U * (1-y^r)/(1-y^p) * y^i < Utilization < U * (1-y^r)/(1-y^p) with U is the max util_avg value = SCHED_CAPACITY_SCALE At a lower capacity, the range becomes: U * C * (1-y^r')/(1-y^p) * y^i' < Utilization < U * C * (1-y^r')/(1-y^p) with C reflecting the compute capacity ratio between current capacity and max capacity. so C tries to compensate changes in (1-y^r') but it can't be accurate. Instead of scaling the contribution value of PELT algo, we should scale the running time. The PELT signal aims to track the amount of computation of tasks and/or rq so it seems more correct to scale the running time to reflect the effective amount of computation done since the last update. In order to be fully invariant, we need to apply the same amount of running time and idle time whatever the current capacity. Because running at lower capacity implies that the task will run longer, we have to ensure that the same amount of idle time will be apply when system becomes idle and no idle time has been "stolen". But reaching the maximum utilization value (SCHED_CAPACITY_SCALE) means that the task is seen as an always-running task whatever the capacity of the CPU (even at max compute capacity). In this case, we can discard this "stolen" idle times which becomes meaningless. In order to achieve this time scaling, a new clock_pelt is created per rq. The increase of this clock scales with current capacity when something is running on rq and synchronizes with clock_task when rq is idle. With this mecanism, we ensure the same running and idle time whatever the current capacity. This also enables to simplify the pelt algorithm by removing all references of uarch and frequency and applying the same contribution to utilization and loads. Furthermore, the scaling is done only once per update of clock (update_rq_clock_task()) instead of during each update of sched_entities and cfs/rt/dl_rq of the rq like the current implementation. This is interesting when cgroup are involved as shown in the results below: On a hikey (octo ARM platform). Performance cpufreq governor and only shallowest c-state to remove variance generated by those power features so we only track the impact of pelt algo. each test runs 16 times ./perf bench sched pipe (higher is better) kernel tip/sched/core + patch ops/seconds ops/seconds diff cgroup root 59648(+/- 0.13%) 59785(+/- 0.24%) +0.23% level1 55570(+/- 0.21%) 56003(+/- 0.24%) +0.78% level2 52100(+/- 0.20%) 52788(+/- 0.22%) +1.32% hackbench -l 1000 (lower is better) kernel tip/sched/core + patch duration(sec) duration(sec) diff cgroup root 4.472(+/- 1.86%) 4.346(+/- 2.74%) -2.80% level1 5.039(+/- 11.05%) 4.662(+/- 7.57%) -7.47% level2 5.195(+/- 10.66%) 4.877(+/- 8.90%) -6.12% The responsivness of PELT is improved when CPU is not running at max capacity with this new algorithm. I have put below some examples of duration to reach some typical load values according to the capacity of the CPU with current implementation and with this patch. Util (%) max capacity half capacity(mainline) half capacity(w/ patch) 972 (95%) 138ms not reachable 276ms 486 (47.5%) 30ms 138ms 60ms 256 (25%) 13ms 32ms 26ms On my hikey (octo ARM platform) with schedutil governor, the time to reach max OPP when starting from a null utilization, decreases from 223ms with current scale invariance down to 121ms with the new algorithm. For this test, I have enable arch_scale_freq for arm64. Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> --- kernel/sched/core.c | 2 +- kernel/sched/deadline.c | 6 ++-- kernel/sched/fair.c | 16 ++++----- kernel/sched/pelt.c | 88 ++++++++++++++++++++++++++++++++++++++++++++----- kernel/sched/pelt.h | 27 +++++++++++++++ kernel/sched/rt.c | 6 ++-- kernel/sched/sched.h | 2 ++ 7 files changed, 123 insertions(+), 24 deletions(-) -- 2.7.4